The invention relates to brushless DC motors and, more particularly, to a start-up process to start a brushless DC motor at any initial speed.
Brushless DC motors are also known as electronically-commutated DC motors wherein the commutation of the stator windings is performed based on the rotor position. Rotor position is essential for commutation in brushless DC motors. However, due to the high manufacturing cost, absolute position sensors to detect rotor position are not desired. To obtain the position information two methods are usually used: single-bit position sensor (optical or magnetic), or sensing the zero crossing points of the winding back EMF. A combination of these two methods is also used in some applications.
To start a brushless DC motor without an absolute position sensor a special start-up scheme is required. Two conventional start-up schemes are: (1) to brake the rotor to a known position and then start running the motor from that position, (2) or energizing the motor windings in a open-loop sequence to bring the motor up to at certain speed and then switch to closed-loop commutation.
In some applications the motor is rotating prior to a command from the control circuitry. Both of the above mentioned conventional start-up schemes have drawbacks in such applications. In the first scheme, the required braking torque is much larger if the rotor is already rotating. The stress on the power electronics is also greater. This is particularly true if the load is a high inertia fan. In the second scheme, a fixed stator-field speed may be too far away from the actual rotor speed and prevent a successful start-up.
Accordingly, there is a need to provide a method of starting a brushless DC motor at any initial speed.
An object of the invention is to fulfill the need referred to above. In accordance with the principles of the present invention, this objective is achieved by providing a method of starting a brushless DC motor at any initial speed. The method (A) establishes an initial stator-field speed and sets a counter of synchronization, (B) measures a speed of a rotor of the motor, (C) compares the speed of the rotor with the stator-field speed to determine if the rotor is synchronized with the stator-field. If the rotor is not synchronized with the stator-field, the method includes (a) re-setting the counter of synchronization, (b) increasing an acceleration portion of motor current (Iacc) to enhance torque, (c) setting the stator-field speed higher than the rotor speed, (d) calculating a period of an open-loop timer, (e) calculating a value for a load portion of the motor current (Ild), where total motor current I=Iacc+Ild, (f) performing commutation based on the open-loop timer and returning to step (B) until the rotor is synchronized with the stator-field. If synchronization of the rotor and stator-field is determined, the method includes incrementing the counter of synchronization, determining if the counter is greater than a certain value, and if the counter is greater than the certain value, switching from open-loop commutation to closed-loop commutation. If the counter is not greater than the certain value, the method includes proceeding to step (c).
Other objects, features and characteristics of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification.